Conventional semiconductor lasers are small discrete devices formed of an active lasing medium confined in a waveguide between front and rear, partly reflective, mirrors. Laser light is emitted out an edge of the active lasing medium.
A great deal of research and development effort has been expended in an attempt to form surface emitting lasers; that is, lasers which emit light perpendicular to the longitudinal plane of the active medium out an upper or lower surface of the laser structure.
One such device is illustrated in U.S. Pat. Nos. 4,784,722 and 4,718,070 to Liau et al.
In the structure of Liau et al., a light transmissive mirror surface is formed adjacent the light emitting edge of a buried-heterostructure (BH) laser. A parabolic mirror is formed adjacent the mirror surface to collimate and redirect light emitted from the edge in a plane perpendicular to the active layer.
In the embodiment of FIG. 7, a pair of 45.degree. light reflective mirrors are fabricated adjacent the light emitting edge of the active layer of a BH laser. The 45.degree. mirrors direct edge-emitted light out the substrate surface at a 90.degree. angle to the plane of the active layer. Two partly light transmissive mirrors are provided between the 45.degree. mirrors and the substrate surface by etched air gaps. These mirrors provide optical feedback to the laser to sustain oscillation within the Fabry-Perot intra-cavity formed between the output mirrors at 106.
The Liau et al. patents also describe a lens formation embodiment (FIGS. 5-6) wherein a collimating lens may be formed on a substrate or, optionally, formed at the bottom of the FIG. 7 substrate to further collimate light emitted from the substrate surface (See col. 9, lines 1-13).
The above described Liau et al. invention solves a number of problems present in conventional laser devices by providing an integrated lens for coupling a surface emitting laser beam to external optics. Conventional semiconductor lasers produce beams with large beam divergence of 30.degree., or more, making coupling to external optics difficult. With a properly aligned monolithic lens system, large refractive index semiconductor material can be used to form high quality large-numerical-aperture lenses which can result in an output beam divergence less than 1.degree. for lens diameters greater than 100 .mu.m.
However, a need still exists for a method and resulting process and laser device in which the lens is accurately aligned with the reflective mirror surface of the laser.